The research plan outlined in this proposal will elucidate the mechanisms by which the insulin-like growth factors, IGF-I and IGF-II, and their primary signaling receptor, the IGF type I receptor (IGF-IR) mediate growth and differentiation of cells in the mammary gland. The goal of the proposal is to test the hypotheses that: 1) the IGFs and IGF-IR are essential for mammary epithelial proliferation and for epithelial-stromal signaling, and 2) IGF-I and IGF-II have differential functions in mediating mammary gland growth and differentiation. Experiments in specific aim one utilize a mammary gland culture system to determine whether IGF signaling regulates cell cycle progression in the mammary gland by regulating expression and activation of both the G1 and G2 checkpoint complexes. These experiments also will elucidate the mechanisms for IGF/EGF interactions in mediating cell cycle progression during mammary development. In the second specific aim, a genetic approach will be used to test the role of IGF-IR signaling in EGF or TGF- mediated growth of mammary glands in vitro and in vivo. Experiments in aim two also will address the role of the IGF-IR in epithelial-stromal signaling in mammary development. In aim three of the research plan, we will utilize additional genetic approaches to test whether IGF-I and IGF-II function differentially in mammary development. These experiments will include studies to determine whether IGF-II expression is directly regulated by progesterone in the mammary gland. The experimental data generated will have a significant impact on our understanding of IGF and IGF receptor function in normal development of this tissue. The IGFs and IGF-IR are among the increasing numbers of genes with proposed roles in both normal breast development and breast cancer, thus, results of these experiments also will provide a basis for understanding mechanisms of IGF action in breast cancer.